Apparatus and method for determining fluid depth

ABSTRACT

A fluid depth determination system is provided for a vessel that contains first and second immiscible fluids, where the second fluid floats on top of the first fluid forming an interface therebetween. The system includes a first pressure probe which is located in the floor of the vessel, a second pressure probe being at least vertically moveable in the first and second fluids in a region of the interface, means for collecting pressure readings from the first and second pressure probes and means for calculating the depth of the first fluid based on a difference in the pressure readings. The present invention is also directed to a method of determining a depth of a first fluid having an immiscible second fluid floating on top of the first fluid and forming an interface therebetween.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 60/552,627 filed Mar. 12, 2004.

BACKGROUND OF THE INVENTION

The present invention is directed to measuring the level or height or depth of a fluid in a container or a vessel. Although the invention can be applied to many situations, the invention is particularly suitable for accurately determining the height of liquid or molten metal in a metallurgical vessel by taking into account the amount of slag on the surface of the liquid metal and taking into account the erosion of the vessel floor.

Most metals are industrially processed from raw materials to finished products involving several stages in their production at high temperatures while in the liquid state. These liquid metals are metallurgically processed, transferred and/or distributed from one stage to another in vessels or containers. Although specially constructed to withstand the high temperatures and sometimes erosive environment of the metal manufacturing industry these containers under-go changes in their protective lining. The protective lining is typically constructed of refractory material which protects the vessel's physical structure from the molten metal. As the liquid metals are metallurgically processed, the refractory material erodes within the vessel. These changes are due to erosive forces, whether chemical, abrasive or a collection of thermal events which are normal in this industry and is provided for by periodic maintenance of these vessels (such as replacing the refractory).

Most metallurgical processes involve the removal of impurities from the liquid metal during some stage of the metal manufacturing process. This removal process creates or is aided by the presence of a second liquid which floats upon the liquid metal and is call slag or dross or other descriptive terms depending upon the type of metal and the process to purify it to a finished stage. This slag is noticeably less dense than the metal it floats upon. These two liquids, for the most part, coexist as immiscible liquids in many metallurgical processes where it would be advantageous to know the quantity of liquid metal held in the vessel for production process reasons.

There are many industries in which the quantity of a liquid in a container and its measurement is desired for production purposes. This need is serviced by many technologies, RF capacitance, conductance, hydrostatic tank gauging, orifice meters, weight of fluid, radar, radio and microwave, and ultrasonics are the leading sensor technologies in liquid level tank measurement and control operations. Each of these technologies are mostly applied to a single room temperature fluid in a static container, that is, the container is not eroding and therefore the point of installation of the detection device is a point of reference to measure the level against. The severe high temperatures involved in the metals processing industry, for example steel, where the temperature in an electric arc furnace will typically be about 1650° C. (3000° F.) at the end of the melting stage, eliminates the practical use of most contact measuring devices, coupled with the presence of a mostly erosive, electrically conductive slag, present numerous challenges for conventional level measuring techniques.

Knowing the height of liquid steel independent of the amount of slag floating upon it within an electric arc furnace is useful in order for the furnace to operate as efficiently as possible. These vessels have a high amount of erosion and a selected weight of metal will have a varying thickness of metal between the vessel bottom and power delivering electrodes. In certain types of electric arc furnaces, operating with not enough molten or liquid metal can damage or destroy a furnace bottom anode. Operating with too much molten metal can damage the roof or sidewalls of the vessel.

Similarly, knowing the level of liquid steel independent of the amount of slag floating upon it in a vessel whose function is to transfer the liquid from one stage to another, for example a ladle, is helpful during the empting process. These vessels erode wider with use so that a set weight of metal is lower in the vessel, when measured from the top down. Also these vessels contain a certain thickness of slat, which adds an unknown thickness to the overall height of fluid contained therein. Typically these vessels are emptied from the bottom. The height of liquid metal above the draining point is important so that the empting process can be interrupted before the coexisting slag is transferred along with the liquid metal.

In tundish operations it is more efficient to operate knowing the level of liquid steel during grade changes for the continuous casting process and at the end of a cast so that slag is prevented from entering the mold. During continuous casting, the amount of slag in the tundish increases. Knowing the weight of metal per volume of the tundish at the start, by the use of load cells does not enable one to determine the amount of slag in the tundish at a given period of time and cannot be corrected for because of the unknown weight added during casting. The lower the level of metal is within the tundish during metal grade changes (the changing of one type of steel with another) the more the operator would benefit from knowing the metal content and not the metal plus slag content.

Accordingly, it would be beneficial to be able to accurately and easily determine the height of such molten metals independent of the amount of slag floating upon it and other liquids in a container or vessel, particularly where the liquid and/or vessel are of such a nature that measurement by visual or optical or conventional means is impossible or impracticable. One of the oldest and most common methods of measuring liquid level is to measure the pressure exerted by a column (or head) of liquid in the vessel. By adapting this technology to the molten metals industry, the level of liquid metal could be obtained.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the invention, a fluid depth determination system is provided for a vessel having a floor and side walls for containing first and second immiscible fluids. The first fluid has a bottom resting on the floor of the vessel and the second fluid floats on top of the first fluid forming an interface therebetween. The system includes a first pressure probe situated adjacent the floor of the vessel and the bottom of the first fluid; a second pressure probe being at least vertically moveable in the first and second fluids in a region of the interface; means for collecting pressure readings from the first and second pressure probes; and means for calculating the depth of the first fluid based on a difference between a pressure reading of the second probe and a pressure reading of the first probe.

According to a preferred embodiment of the fluid depth determination system, the first pressure probe has a gas conduit or orifice located in the floor of the vessel, where the conduit is erodable with the floor of the vessel, such that the open end of the gas conduit remains located in the vessel floor. The second pressure probe also includes a gas conduit with an open end that is at least vertically movable in the first and second fluids in a region of the interface.

According to another aspect of the invention, a method is provided for determining the depth of a first fluid having an immiscible second fluid floating on top of the first fluid and forming an interface therebetween. The method includes the steps of measuring a first pressure at a height adjacent the bottom of the first fluid; measuring a second pressure at the height of the interface between the first and second fluids; and then calculating the depth of the first fluid based on the difference in the first and second pressures, using the physical relationship of the height, h, of the fluid being equal to the pressure, P, of the fluid divided by the density, ρ, of the fluid and the acceleration due to gravity, g.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

FIG. 1 is a diagrammatic view of one embodiment of the depth determination system according to the present invention;

FIG. 2 is a schematic view of a portion of the system according to the embodiment of FIG. 1;

FIG. 2 a is an enlarged detail view in perspective of the Area IIa in FIG. 2; and

FIGS. 3 a through 3 f are schematic diagrams illustrating the operation of the second pressure probe according to a depth determination method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an apparatus and a method for measuring the depth or height of a fluid when there are two fluids in a container or vessel that are immiscible and have different densities.

The pressure that a fluid exerts in a container is directly proportional to the depth of the fluid. In other words, as the pressure is monitored from the bottom of a fluid to the top of the fluid, the pressure decreases. An equation that expresses this principle is: P=ρgh,  Fomiula (I)

-   -   where P is the pressure, ρ is the density of the fluid, g is the         acceleration due to gravity, and h is the depth of the fluid.

The present invention utilizes this relationship in order to calculate a depth or a height of a first fluid in a situation where there are at least two fluids. The at least two fluids have different densities and are generally immiscible. Therefore, a second fluid would float on top of the first fluid, and if there is a third fluid, it would float on top of the second. Where the first fluid and the second fluid meet, all interface is typically created. The first fluid has a bottom resting on the floor of the vessel. Determining the depth, h of the first fluid is accomplished by measuring a first pressure at or near the bottom of the first fluid and then measuring a second pressure at or near the interface, where the first fluid and the second fluid meet and then using the difference between the first and the second pressures to calculate the depth, h, of the first fluid. The difference between the two pressures can be determined by being calculated mathematically and measuring each pressure separately, or by using a differential pressure transducer, either method is acceptable without deviating from the principle of the invention.

The system and method of the present invention are suited for many operations, including metallurgical, where the first fluid comprises molten or liquid metal and the second fluid comprises slag. However, the invention is not limited to this operation and has many applications, especially where the depth, h, of a first fluid is not easily obtained by customary means such as a sight glass or a gauge.

The way of measuring the pressure is not limited to any particular means or method. I-low the pressure is measured or the technique or apparatus used is not important. The present invention is not limited to any particular pressure sensor or pressure probe type. Also, for ease of explanation, the present invention will be described hereinafter with particular reference to determining the depth of molten metal in an electric arc furnace, it being understood that the invention is not limited thereto.

Referring to FIG. 1, the system is designed to accurately determine the depth, h, of molten metal 10, especially in applications where the floor 12 of a vessel 14 erodes. The vessel, as defined herein, has sidewalls 16 and a floor 12 that contain the molten metal 10 and the slag 18. The vessel may also have a roof, for example in applications where the vessel is an electric arc furnace (EAF). Examples of other metallurgical vessels and operations where the system and method of the present invention can be used include metallurgical ladles, basic oxygen furnaces, argon-oxygen decarburization, ladle metallurgical facilities, tundish operations, and other various holding vessels.

The system includes two pressure probes 20, 22. The first pressure probe 20, referred to hereinafter as the bottom probe 20, measures the pressure at the bottom 24 of the molten metal (including the combined pressures of the first and second fluids, i.e. molten metal or slag) and can be stationary. In a preferred embodiment, the bottom pressure sensor is stationary in so far as understanding that it will erode at the same rate as the floor of the vessel erodes. The second pressure probe 22, hereinafter referred to as the interface probe 22, detects the pressure at or near the interface 26 of the slag and molten metal and must be moveable. In a preferred embodiment of the invention, the interface probe is moveable.

In an embodiment of the present invention, the bottom pressure probe comprises a gas conduit 28 that comprises at least one tubular member 28. The at least one tubular member 28 possesses at least one orifice 30 or is open on one end 30 of the at least one tubular member 28. The other end of the at least one tubular member 28 is connected to a pressure sensor 32, at least one valve 34, and a gas source 36. Similarly, the second pressure probe 22 comprises a gas conduit 38 that is constructed of at least one tubular member 38 that possesses at least one orifice 40 or is open on one end 40 of the at least one tubular member 38. The other end of the at least one tubular member 38 is connected to a pressure sensor 42, at least one valve 44, and a gas source 36. The gas source need not be shared and each probe can have its own source.

In another embodiment of the invention, the at least one tubular member of the bottom pressure probe can be replaced by having a hole or orifice formed in the bottom of the vessel which would then be connected to a pressure sensor 32, at least one valve 34, and a gas source 6. The hole or orifice can be bored through the bottom of the vessel and the refractory and/or be formed from a mold when the refractory liner is being poured.

The system also includes means 32, 42 for collecting pressure readings from the bottom pressure probe and the interface pressure probe and means 46 for calculating the depth of the first fluid, which are described in more detail below.

In one embodiment of the present invention, the at least one tubular member 28 of the bottom pressure probe is embedded in the floor of the vessel in such a way that the open end of the tubular member is substantially flush with the floor of the vessel.

The diameter of the tubular member(s) of the bottom pressure probe is not limited to any size for each tubular member and it depends on the application. In an embodiment of the present invention, the diameter of the at least one tubular member can be in a range of about 0.25 mm to about bout 5 mm for each tubular member. In a preferred embodiment of the present invention, the diameter of the at least one orifice is about 0.5 mm to about 2 mm for each tubular member.

In the present invention the at least one tubular member of the bottom pressure probe can be constructed out of any desired material. Examples include ceramic, and/or various types of steels or other metals depending on the type of metal measured. One consideration is that in some molten metal operations the floor of the vessel erodes as the operation progresses. In order to ensure that the at least one tube is flush with the floor, a metal that liquefies or dissolves at the maximum service temperature may be the appropriate material choice. In the metallurgical field, a metal of a higher melting temperature may dissolve into a metal of a lower melting temperature by entering as a solution with the first metal much the same as sugar dissolving in coffee. The selection of the metal is not only dependent upon its melting temperature but also it solvency in the measured liquid metal and another consideration is if the purity of the liquid metal is to be obtained. If the at least one tubular member is constructed out of a liquating or dissolving metal, the metal will melt as the floor of the vessel erodes, thereby ensuring that the open end(s) or orifice(s) of the bottom pressure probe remain(s) flush with the floor of the vessel, and an accurate depth or height of the liquid metal is measured. One example includes an at least one tubular member made of a metal such as stainless steel where the vessel is an EAF.

The tubular member-(s) for either or both of the probes is(are) generally cylindrical in nature and the sidewalls of the tubular member, are generally constructed so that the transverse cross-section of the tubular member is substantially circular. However, the transverse cross-section can comprise any shape, for example: substantially square, substantially pentagonal, or the like.

Regarding, the bottom pressure probe, one embodiment of the system includes the at least one tubular member 28 being constructed of three tubular members placed very close in proximity to each other. The three tubular members may be next to each other so that they are touching, as in FIG. 2 and FIG. 2 a. In an embodiment of the present invention, each of the three tubular members share a gas source, although it is not necessary for the operation of the present invention. Also, a supportive substance, material or device may be placed around the three tubular members. The three tubular members need not each be constructed of the same material, although it is preferred in some operations. The diameters of the three tubes need not be the same diameter. The diameter of the orifice or open end of each of the tubes ranges from about 0.12 to about 15 mill. In a preferred embodiment, the diameter of each of the tubes is about 0.5 mm to about 2 mm. In a preferred embodiment, the open ends or orifices of the respective tubes are substantially flush with the floor of the vessel. The benefit of having three orifices close together when measuring pressure is that the bubbles 48 that form at the open end of each of the three tubular members 24 will disturb each other, leading to easier bubble detachment from the open end of the tubular structures. The construction of the bottom pressure probe is such that the flow of gas is not impaired or appreciably restricted in its release from the bubbling tubular members except for the restraining pressure of the metal fluid above it. The opening(s) are chosen such that a change in the length of the tubular member(s) 28 due to eroding are negligible in comparison to the restraining pressure of the fluid.

There is no specific way the bottom pressure probe is to be mounted in the vessel and it can vary from vessel to vessel and application to application. Typically, the floor of the vessel will have several layers. The outer layer, furthest from the metal, is comprised of a brick. As you move closer to the metal, the layers are cast, rammed, or gunned and may be poured into place and set by heat. In these types of layers the material flows around the tube(s) and after setting they are fixed in place.

Regarding the pressure measuring means 32 for the bottom pressure probe, any means known by those skilled in the art can be used. For example, any off the shelf product can be used or it may be constructed or a combination thereof, depending upon the application. A non-limiting example of an off the shelf product is a flush diaphragm transducer model 211 B-SZ 10/AA available from the GP:50 Company. The pressure measuring means should be made of suitable materials for the application to prevent corrosion and premature wear. The pressure measuring means can include a visual display, such as a gauge. The pressure measuring means 32 communicates with a means 46 for calculating the depth such as a simple computer programmed with the pressure density Formula I. The means for calculating the depth may receive the data from the pressure measuring means via electrical signals from the pressure measuring means.

The valve of the bottom pressure probe can be any off the shelf product or it may be constructed, or even a combination thereof. The valve can be any type of shut off or flow control or pressure control device known in the art and should be made of suitable materials for the application to prevent corrosion and premature wear. There can be more than one valve for the operation of the bottom pressure probe. For example, the bottom pressure probe can be equipped with safety pressure relief valves or other valves. Any or all of the valves used can be either manually operated or computer operated, or a combination thereof.

The gas flow in the bottom pressure probe is controlled using a regulator 39 to an appropriate amount in order to prevent the intrusion of molten metal into the tubular member. One skilled in the art would realize that this is a function of the fluid weight above it, and the liquid metal properties, such as the viscosity, surface tension, etc. Therefore, the amount of flow or bubbles emitted will change from application to application. In one embodiment of the present invention the gas flow is about 30 to about 240 cc/minute. The example of a regulator that can be used for such all application is an A Series RMA Rate-Master Flowmeter available from Omega Engineering.

The interface pressure probe detects pressure at or adjacent the region of the interface 26. In one embodiment of the system, the interface pressure probe is moveable. In an embodiment of the invention, the interface pressure probe comprises at least one opening 40 for emitting gas bubbles. The opening 40 can be of any orientation. For example, up, down or sideways. In an embodiment of the invention, the at least one opening 40 is oriented in a substantially sideways position. The at least one tubular member for the interface pressure probe may be constructed out of any suitable material needed for the particular application. In molten steel applications examples of suitable materials include ceramic, stainless steel, refractory ceramics, fused silica, et cetera. The at least one tubular member of the interface pressure probe can be straight or curved. The interface pressure probe may be moveable either manually or by machine or with the aid of a computer or by any mechanical means. The tubular member of the interface pressure probe can be of any suitable construction. The tubular member of the interface pressure probe may be a detachable, expendable device provided with a connecting means, a gas orifice, and protective means as typified by U.S. Pat. No. 5,421,215, which is incorporated herein in its entirety. Also, the probe can be cooled with a type of cooling jacket, if needed. In a preferred embodiment, the tubular member is constructed out of fused silica.

In an embodiment of the invention for the interface pressure probe, the at least one tubular member 38 is connected, to or is in gas communication with, a pressure measurement means 42. The pressure measurement means 42 is connected to, or is in gas communication with, at least one valve 44 and one gas flow regulator 41. Both 41, 42 can communicate electronically to the means 46 for calculating the depth. The pressure measurement means 42 can also be connected to a means for calculating the depth, h. The valve 44 is then connected to a gas source 36. The gas source 36 may be shared with the bottom pressure probe or each probe can each have its own respective gas source.

Regarding the pressure measuring means 42 for the interface pressure probe, any means may be used that is known by those skilled in the art and can be any device or series of devices that are described for the bottom probe, above. Although, each probe's equipment is independently selected and need not be exactly the same.

The valve 44 of the interface pressure probe can be any off the shelf product or it may be constructed as is described for the bottom probe. Although, each probe's equipment is independently selected and need not be the same.

Using a regulator 45, the gas pressure in the interface pressure probe is controlled at various pressures, depending on the application. An example of a range is from about 20 kPa to about 50 kPa. In an embodiment of the invention, the back pressure of the fluid on the gas is measured and approximately 7 to 45 kPa gauge pressure is desired to prevent the intrusion of molten metal into the tube. The amount of pressure used would be dependant on the above fluid weight, viscosity, surface tension, etc.

The gas that can be used is not limited to any particular type or kind. The type of gas used depends on the fluids and whether the fluids will react with the gas. Examples of gases that can be used include nitrogen, air, and inert gases, such as argon or helium. Argon is preferred gas when operating in conditions where it is preferable to not contaminate the liquid or molten metal.

The calculating means 46 can be any device known in the art. Examples include any type of computational devices, computer, any combination of these, et cetera. Also, the depth can be calculated by hand. The depth can be calculated in any way known in the art. Methods of calculating the depth is discussed in detail below. The calculating means can also include or work in conjunction with a means for collecting pressure measurements or readings. The means for collecting pressure readings can be various devices known in the art. Examples include any type of recording device, any type of graphing device, a computer and combinations thereof, et cetera. Also, the pressure readings can be collected manually or by memory. Another example can include a computer that records the pressure readings and then simultaneously calculates the fluid depth.

The present invention also describes a method of determining the depth of a first fluid, having an immiscible second fluid floating on top of the first fluid and forming an interface therebetween. The method includes: measuring a first pressure at a height adjacent a bottom of the first fluid, measuring a second pressure at a height of the interface between the first and second fluids, and calculating the depth, h, of the first fluid based on the difference in the first and second pressures.

The method of the present invention is suited for many operations, including those where the depth of a first fluid may not be easily obtained, such as in metallurgical operations. The method will be described using the system terminology from above and so the first fluid will be referred to as molten metal and the second fluid will be referred to as slag.

At least two kinds of pressure can be measured: 1.) the pressure that is required to create a bubble at the at least one orifice or the opening of each of the tubular members, and 2.) the back pressure that is imposed by the fluid. Measuring the back pressure or the pressure needed to overcome the force exerted by the weight of the fluid to allow free flow is preferred. Gas pressure must be high enough so as to prevent molten metal from entering the tubular member.

In one embodiment of the invention, at least one of the first and second pressure measuring steps comprises emitting at least one gas bubble at the height of the measurement and measuring a pressure of a gas flow which forms the bubble.

In an embodiment of the present invention, the bottom pressure probe comprises an orifice that is located in the floor of the vessel and the floor of the vessel erodes over a period of time. This allows the open end 30 of the tubular member to remain flush with the floor of the vessel and increases the accuracy of the measurement of the first pressure, thereby increasing the accuracy of the depth calculation.

The interface is generally determined by having the interface pressure probe immersed in the molten metal. Then, while slowly moving the interface pressure probe in an upwards direction, preferably at a constant speed, pressure readings are recorded from the interface pressure probe. These readings are observed to determine at what point, in the vertical direction, the greatest change in pressure occurs. The point where the greatest change in pressure occurs indicates an interface, because of the change in density between the two fluids.

In one embodiment of the invention, the location of the interface is determined by steps that are generally depicted in FIGS. 3 a through 3 f. In FIG. 3 a the interface pressure probe has the gas source turned on as it is being immersed through the slag and into the molten metal (FIG. 3 b). The gas flow is then reduced allowing fewer bubbles to be emitted (FIG. 3 c). The interface pressure probe is then moved at least vertically upwards at a predetermined speed, preferably at a slow constant speed (FIG. 3 d). As the interface pressure probe is moved upwards, more bubbles may escape due to the decrease in fluid pressure exerted. As the second probe moves generally vertically upwards, the pressure is measured it is recorded or watched. The interface is located generally at the point where a change in gradient occurs or where the greatest change in pressure occurs, which is due to the change in densities between the molten metal and the slag (FIG. 3 e). The interface pressure probe may then be kept in the same general location as the interface or moved about or even taken out of the fluids.

Calculating the depth of the molten metal can be accomplished using various techniques. The technique described herein is merely an example and the invention should not be limited to the technique described herein. One way is by using the relationship expressed in the equation h=P/ρg, where h is a depth of the molten metal, P is the pressure, ρis the density of the molten metal, and g is the acceleration due to gravity. The height can be calculated by using the equation h=P/ρg, and subtracting the pressure contribution of the slag from the bottom pressure reading. This particular way is accomplished by subtracting the interface pressure reading from the bottom pressure reading and the value obtained is inserted as the pressure, P, in the equation. The density, ρ, of the molten metal is inserted into the equation along with the acceleration due to gravity, g. After accounting for any unit inconsistencies, the height, h, is then calculated. Without departing from the invention, it is evident that because the density of the fluids involved are related to their temperature, bubble formation is dependant upon surface tension and the like; additional mathematical and physical relationships can be used to improve the predictability of the liquid metal depth without violating the principle of the measurement.

EXAMPLE

A non-limiting example describing how the interface is located follows. The interface is generally determined by having the interface pressure sensor immersed in the molten metal. The pressure sensor 38 and gas conduit 22 are regulated by a pressure measuring/regulating means 42. Once immersed into the molten bath to a depth greater than the height of the second fluid (i.e., below the interface), and noticeably into the first fluid, valve 44 is closed. Valve 44 maybe be automatically closed by monitoring the backpressure from gauge 42. The distance of immersion into the first fluid is not important for the measurement. Once the valve 44 is closed, the pressure inside 38 and 22 will come to a pressure equilibrium. The gas pressure at 42 will equal and be offset by the pressure exerted by both the depth into the first fluid of an unknown distance and the entire unknown thickness of the second fluid. This equilibration will be on the order of seconds. After approximately 3 seconds for initial 45 kPa pressurization, the inrtface pressure sensor is withdrawn from the immersion. Slowly removing the interface pressure sensor in an upwards direction, preferably at a constant speed, pressure is relieved through the open end 40 of the tubular member and pressure readings are measured or detected at the pressure measuring means 42 from the interface pressure sensor. The pressure detected at the pressure measurement means 42 will decline proportional to the pressure exerted at the open end of the tubular member by both fluids.

The rate of change of pressure at the pressure measurement means 42 is dependent upon the rate of removal and the density of the fluid. Since the density of the first fluid, in this example, steel, is approximately 7 g/cc, and the density of the second fluid, slag, is approximately 3 g/cc, the pressure drop for the equivalent distance will be approximately twice as great while moving through the liquid metal as compared to moving through the slag. In other words, the rate of pressure drop at the moment the interface sensor orifice 40 moves from the steel to the slag changes by one half. This is a noticeable difference and can easily be detected. These readings are observed to determine at what point, while moving in the vertical direction, the greatest change in the rate of pressure drop occurs. The point where the greatest change in the rate of pressure drop occurs indicates the interface. The back pressure at the moment the interface is detected is recorded. The pressure recorded is the pressure exerted by the quantity of the slag present and is available by the above means, independent of a reference point recording the position of the open end 40 of the tubular member at around or through the interface. No measurement of the thickness of the slag is required.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 

1. A fluid depth determination system comprising a vessel having a floor and side walls for containing first and second immiscible fluids, wherein the first fluid has a bottom resting on the floor of the vessel and the second fluid floats on top of the first fluid fonming an interface therebetween, a first pressure probe situated adjacent the floor of the vessel and the bottom of the first fluid, a second pressure probe being at least vertically moveable in the first and second fluids in a region of the interface, means for collecting pressure readings from the first and second pressure probes, and means for calculating the depth of the first fluid by determining a difference in a pressure reading of the second probe and a pressure reading of the first probe.
 2. A system according to claim 1, wherein the vessel comprises a metallurgical vessel, the first fluid comprises molten metal, and the second fluid comprises slag.
 3. A system according to claim 2, wherein the metallurgical vessel is selected from the group consisting of metallurgical ladles, basic oxygen furnaces, argon-oxygen decarburization vessels, and tundish operation vessels.
 4. A system according to claim 1, wherein at least one of the first and second pressure probes comprises at least one orifice for emitting gas bubbles, a source of gas supplying a gas flow to the orifice, and a means for measuring pressure of the gas flow as it forms bubbles at the orifice.
 5. A system according to claim 4, wherein both the first and second pressure probes have at least one orifice for emitting gas bubbles, a source of gas supplying a gas flow to the orifice, and a means for measuring pressure of the gas flow as bubbles form at the orifice.
 6. A system according to claim 4, wherein the at least one orifice of the first pressure probe comprises at least one orifice located in the floor of the vessel.
 7. A system according to claim 4, wherein the at least one orifice of the second pressure probe is found at an end of at least one tube which is immersable in the first and second fluids.
 8. A system according to claim 5, wherein the first and second pressure probes have a common gas source.
 9. A system according to claim 5, wherein the first pressure probe comprises three orifices for emitting gas bubbles, a source of gas supplying a gas flow to the orifice, and a means for measuring the pressure of the gas flow, wherein the three orifices are found wherein the threes orifices are each found at an end of a tube and the tubes are substantially next to each other and are embedded in the floor of the vessel in a substantially vertical position so that the three orifices are substantially flush with the floor of the vessel.
 10. A system according to claim 6, wherein the at least one orifice is formed at the end of at least one tube through the floor of the vessel.
 11. A system according to claim 10, wherein the at least one tube is erodable with the floor of the vessel such that the at least one orifice remains located in the floor.
 12. A method of determining the depth of a first fluid having an immiscible second fluid floating on top of the first fluid and forming an interface therebetween, the method comprising measuring a first pressure at a height adjacent a bottom of the first fluid, measuring a second pressure at a height of the interface between the first and second fluids, and calculating the depth of the first fluid based on the difference in the first and second pressures and using a formula: h=P/ρg  (I) wherein P=pressure, ρ=density of the fluid, g=acceleration due to gravity, and h=depth of fluid.
 13. The method according to claim 12, wherein the calculating step comprises subtracting the second pressure from the first pressure and applying the formula (I) to the difference obtained.
 14. The method according to claim 12, wherein at least one of the first and second pressure measuring steps comprises emitting at least one gas bubble at the height of the measurement and measuring a pressure of a gas flow which forms the bubble.
 15. The method according to claim 12, wherein the steps of measuring the second pressure comprises immersing a second pressure probe in the first fluid below the interface, withdrawing the second pressure probe from the first fluid at a predetermined speed, recording the pressures of the first and second fluids as the second pressure probe crosses the interface, and determining, a change in gradient.
 16. The method according to claim 12, wherein the first fluid and second fluid are contained in a vessel having sidewalls and a floor wherein the floor erodes and the first pressure is measured where the bottom of the first fluid and the floor of the vessel are in contact.
 17. The method according to claim 12, wherein both the first and second pressure probes have an orifice for emitting gas bubbles, a source of gas supplying flow to the orifice, wherein a back pressure is created from the fluid in the orifice and the back pressure is what is measured.
 18. The method according to claim 14, wherein the at least one measuring step measures the pressure of the gas which is just necessary to overcome the back pressure of the fluid.
 19. A fluid depth determination system comprising a vessel having a floor and sidewalls for containing first and second immiscible fluids, wherein the first fluid has a bottom resting on the floor of the vessel and the second fluid floats on top of the first fluid forming an interface therebetween, a first pressure probe comprises at least one orifice located in the floor of the vessel, wherein the at least one orifice is located at the end of at least one tube and the at least one tube is erodable with the floor of the vessel such that the at least one orifice remains located in the floor, a second pressure probe being at least vertically moveable in the first and second fluids in a region of the interface, means for measuring pressure readings; a gas source; and a means for calculating, the depth of the first fluid based on a difference in the pressure readings.
 20. A method of determining a depth of a first fluid having, an immiscible second fluid floating on top of the first fluid and forming an interface therebetween, the method comprising, measuring a first pressure at a height adjacent a bottom of the first fluid, wherein the first pressure is a measurement of a back pressure necessary to overcome a back pressure of the fluid, measuring the second pressure at a height of interface between the first and second fluids comprising immersing a second pressure probe in the first fluid below the interface, withdrawing the second pressure from the first fluid at a predetermined speed, analyzing the pressures of the first and second fluids as the second pressure probe crosses the interface and determining where a greatest change in pressure occurs; wherein the second pressure is a measurement of a pressure necessary to overcome a back pressure of the fluid, calculating the depth of the first fluid based on the difference in the first and second pressures using the formula: h=P/ρg  (I) wherein P=pressure, ρ=density of the fluid, g=acceleration due to gravity and h=depth of the fluid. 